Suspiciously letting one off ain’t him (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Anagram (suspiciously) of AIN’T H [i] M , removing (letting off) an I (one in Roman numerals). Clue-as-definition: a hitman doesn’t generally let his victims get away. Or perhaps we have to imagine him “letting off” a shot from a firearm, but in a way that doesn’t raise suspicion about him.